1. Field of the Invention
A hardware problem can arise in the economical implementation of a keyswitch detect means in a keyboard operated electronic musical instrument which is constructed with microelectronic circuits. This problem is one of interfacing signal leads from a large number of keyswitches to a limited number of leads that are usually available in microelectronic circuit packages.
One approach that has been employed to deal with the multiple lead interfacing problem is to use time division multiplexing of the keyswitch states. This technique is implemented by assigning a unique time slot to each keyboard switch. The state of each keyswitch is indicated by the presence or absence of a signal pulse in the time slot assigned to the switch. An advantage of time division multiplexing is that the entire status of all the instrument's keyboard switches can be transmitted on a single signal wire. A negative attribute of time division multiplexing is that the scanning, or search, time for the multiplicity of keyswitches is fixed and is independent of the number of keyswitches that have been closed, or actuated. Such a fixed scan time can be a detrimental attribute of a keyswitch detect system because the wasted time used to sequentially scan an entire array of keyswitches when only a limited small number could possibly be actuated can result in a loss or delay of a keyswitch state change detection when the musician plays very fast. A time division multiplex note detection system is described in U.S. Pat. No. 3,610,799.
A time division multiplex system for a keyboard operated electronic musical instrument is described in U.S. Pat. No. 4,022,098. In the system described in this patent provision is made for intermanual coupling. By using pulse controlled sequential connection of manual coupling and keyboard switches an economy in wiring is obtained and keys operated on one keyboard can cause tones to sound which are associated with the same or another keyboard.
A keyswitch detect and assignor system for a keyboard operated electronic musical instrument is described in U.S. Pat. No. 4,022,098. In the described system the keyboard switches are arranged in groups of P switches, with Q groups per set of switches, and S sets of switches per keying system. During a search cycle, each group of P switches is sequentially examined to detect a change in a switch state of any member since the preceding search cycle. A change in a switch state causes the system to enter into an assign node cycle during which a data word is created in memory corresponding to a newly detected switch closure, or is removed from memory corresponding to a newly detected switch opening. After an assignment of a newly detected switch closure has been completed the system returns to the search cycle.